For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load,  is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column  supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Monostrand bonded systems are used when forming long post-tensioned concrete structures, or those which must carry especially heavy loads, such as elongated concrete beams for buildings, bridges, highway passes, etc. A single strand of cable is placed within a duct and then cemented therein in order to achieve the required compressive forces for offsetting the anticipated load. Such bonded monostrand post-tension systems effectively retain the cable in a cemented environment such  that if the tension is somehow released from the ends of the tendon, the required compressive forces will still be retained by the cemented tendon. As a result, such bonded monostrand systems are often used where the concrete structure may, at some time in the future, be reduced in size, cut, or otherwise manipulated so as to detach the tensioning anchor from the remainder of the tendon.
In such monostrand systems, it is highly desirable to protect the tensioned steel cable (or tendon) from corrosive elements, such as de-icing chemicals, sea water, brackish water, and even rain water which could enter through cracks or pores in the concrete and eventually cause corrosion and loss of tension of the cable. In such monostrand systems, the cable typically is protected against exposure to corrosive elements by surrounding it with a metal duct or, more recently, with a flexible duct made of an impermeable material, such as plastic. The protective duct extends between the anchors and in surrounding relationship to the cable. Flexible duct, which typically is provided in 20 to 40 foot sections, is sealed at each end to an anchor and between adjacent sections of duct to provide a water-tight channel. Grout then may be pumped into the interior of the duct in surrounding relationship to the cable to provide further protection.
Various patents have issued, in the past, for devices relating to such duct assemblies. For example, U.S. Design Pat. No. 400,670, issued on Nov. 3, 1998, to the present inventor, shows a design of a duct. This duct design includes a tubular body with a plurality of corrugations extending outwardly therefrom. This tubular duct is presently manufactured and sold by General Technologies, Inc. of Stafford, Tex., the licensee of the present inventor.
The present inventor is also the inventor of U.S. Pat. No. 5,474,335, issued on Dec. 12, 1995. This patent describes a duct coupler for joining and sealing between adjacent sections of duct. The coupler includes a body and a flexible cantilevered section on the end of the body. This  flexible cantilevered section is adapted to pass over annular protrusions on the duct. Locking rings are used to lock the flexible cantilevered sections into position so as to lock the coupler onto the duct.
U.S. Pat. No. 5,762,300, issued on Jun. 9, 1998 to the present inventor, describes a tendon-receiving duct support apparatus. This duct support apparatus is used for supporting a tendon-receiving duct. This support apparatus includes a cradle for receiving an exterior surface of a duct therein and a clamp connected to the cradle and extending therebelow for attachment to an underlying object. The cradle is a generally U-shaped member having a length greater than a width of the underlying object received by the clamp. The cradle and the clamp are integrally formed together of a polymeric material. The underlying object to which the clamp is connected is a chair or a rebar.
U.S. Pat. No. 5,954,373, issued on Sep. 21, 1999 to the present inventor, shows another duct coupler apparatus for use with ducts on a multi-strand post-tensioning system. The coupler includes a tubular body with an interior passageway between a first open end and a second open end. A shoulder is formed within the tubular body between the open ends. A seal is connected to the shoulder so as to form a liquid-tight seal with a duct received within one of the open ends. A compression device is hingedly connected to the tubular body for urging the duct into compressive contact with the seal. The compression device has a portion extending exterior of the tubular body.
U.S. patent application Ser. No. 09/752,126, filed on Dec. 29, 2000 by the present inventor, describes a tendon-receiving duct having longitudinal channels. In this patent application, two forms of the duct are described. One form has a generally circular cross section and the other form has a generally oval cross section. The longitudinal channels are evenly spaced around the duct. These longitudinal channels extend for the length of the duct and allow each of the corrugations to connect with each other. As a result, when grout is introduced into the interior passageway of the  duct, it will flow between the corrugations along the longitudinal channels so as to effectively fill the interior of the duct with grout. The longitudinal channels provide a certain amount of rigidity and straightness to the duct so as to facilitate installation of a plurality of cables (ro tendons) therein. This duct is not particularly designed for use with monostrand systems.
It is an object of the present invention to provided a tendon-receiving duct which can accommodate only a single tendon therein.
It is another object of the present invention to provided a tendon-receiving duct which facilitates the flow of grout throughout the spaces in the interior passageway of the duct between the surfaces of the tendon and the inner wall of the duct.
It is a further object of the present invention to provided a tendon-receiving duct which improves the rigidity of the duct in the longitudinal direction.
It is a further object of the present invention to provided a tendon-receiving duct which facilitates the ability to install the single tendon within duct.
It is a further object of the present invention to provided a tendon-receiving duct which reduces undulations along the length of the duct.
It is a further object of the present invention to provided a tendon-receiving duct which minimizes restrictions during the installation of the tendon on the interior of the duct.
It is a further object of the present invention to provided a tendon-receiving duct which is easy to manufacture, easy to use and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims. 